I want to add a small extension in an area 1.8 x 3m on the side of my house, primarily as use as a utility and storage room. Access would be via an existing external door. Use of double skin brick walls with cavity would make the interior far too small so I was thinking of using wood frame construction with Kingspan-type insulation and plasterboard on top. How thick would such a wall need to be to meet building regs? (The floor and roof will be similarly constructed). I was thinking 100mm thick.